Recently, high performance requirements for internal combustion engines have increased. Mutually offsetting problems arise such that if the reduction of harmful exhaust gas is achieved, the engine output power is reduced and if the fuel consumption is conserved, engine output power is reduced. Therefore, it is required that the above-described mutually offsetting problems be solved to insure high performance while improving engine efficiency. One example of this problem can be exemplified in the case of engine idling. If engine vibrations at the time of idling can be reduced an improvement in the stability of engine operation can be achieved.
A Japanese Patent Application First Publication (unexamined) Showa 59-201936 published on Nov. 15, 1984 exemplifies a previously proposed combustion state controlling system for an internal combustion engine in which engine rotational speed is detected during the combustion stroke of each cylinder during engine idling, the rotation speeds of the respective engine cylinders are compared with the average engine revolution speed for all engine cylinders, and the fuel injection quantity for each cylinder is corrected so as to make the former approach the latter, in order to reduce engine vibrations.
A Japanese Utility Model Application First Publication (unexamined) showa 63-198473 published on Dec. 21, 1988 exemplifies another previously proposed combustion state controlling system in which the ignition timing for each cylinder is corrected rather than the fuel injection quantity described above.
The two above-described previously proposed combustion state controlling systems for internal combustion engines determine a correction quantity for each engine cylinder according to a difference between the engine rotation speed for each cylinder and average value of the rotational speed for all cylinders. Therefore, the correction quantities for all cylinders are uniformly deviated and the combustion state when the correction is carried out is controlled at a driving point which has been deviated from a target air-fuel mixture ratio or ignition timing determined on the basis of optimal cleanness of exhaust gas and engine driveability. That is to say, since a total sum of the correction quantity for each cylinder is not corrected so as to become equal to a target value, the air-fuel mixture ratio for all engine cylinders does not coincide with a target air-fuel mixture ratio. Consequently, the cleanness of the exhaust gas is reduced and idling engine revolutions are reduced due to the corrected ignition timing. Therefore, various improvements are desirable to overcome the above-described drawbacks.